Waveguide etch method for multi-layer optical devices

ABSTRACT

An optical device and a method of manufacturing an optical device, including a ridge waveguide second, and a strip-loaded ridge waveguide section, comprises applying two different protective layers and two separate etches at two different depths. The protective layers overlap to protect the same section of the optical device, and to limit the surfaces of optical device to exposure to multiple etches, except at edges where the protective layers overlap.

TECHNICAL FIELD

The present invention relates to a multi-layer optical device, and inparticular to a method of manufacturing a multi-layer optical device,which attempts to limit the surfaces of the optical device to a singleetch step.

BACKGROUND

Challenges occur in the guiding of light from one type of waveguide toanother, for example, from a ridge waveguide to a so-called“strip-loaded” ridge waveguide. A ridge waveguide involves an isolatedwaveguide rectangle, while a strip-loaded ridge waveguide comprises amulti-layer section with at least one fairly thin waveguide contactregion connected to one or both sides of a central waveguide rectangle.

A key challenge in waveguide photonics is building waveguides withdifferent depths of strip-loading or ridge waveguides withoutstrip-loading at all, all in the same process. Strip-loading refers to asmall connection of waveguide that is left at the edge of a ridgewaveguide, typically for electrical contact. The typical approach is toutilize multiple etch steps in series. Different parts of the waveguideare exposed at different times. In locations where deeper or evencomplete etching surrounding the waveguides is available, the waveguidewill usually have been exposed to multiple etches. This leads to“interface effects” on the edge of the waveguide where multiple etchsteps have been executed, which can lead to performance problems.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing a multi-layer optical device and a method ofmanufacturing a multi-layer optical device, which minimizes locationsundergoing multiple etches.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a method of manufacturinga multi-layer optical device including a ridge waveguide sectioncomprising waveguide material at an upper level, and a strip-loadedridge waveguide section comprising portions at an intermediate level andportions at the upper level, the method comprising:

a) providing a substrate with a layer of the waveguide material thereon;

b) applying a first protective shield over a first area of the waveguidematerial including the portions at the upper level of the strip-loadedridge waveguide section and the ridge waveguide section;

c) applying a first partial etch over a second area of the waveguidematerial including the portions at the intermediate level to etch thesecond area wider than the portions at the intermediate level to definethe portions at the upper level of the strip-loaded ridge waveguidesection;

d) applying a second protective shield over a third area of thewaveguide material covering the portions at the intermediate level toprotect them from further etching, and the portion at the upper level ofthe strip-loaded ridge waveguide section and the ridge waveguidesection; and

e) applying a second full etch, deeper than the first partial etch, overa fourth area of the waveguide material to define the ridge waveguidesection and the portions at the intermediate level of the strip-loadedridge waveguide section;

wherein the first protective shield overlaps the second protectiveshield, whereby side surfaces of the strip-loaded ridge waveguidesection and the ridge waveguide section are only subject to a singleetch, except at intersecting edges where the ridge waveguide sectionintersects the strip-loaded waveguide section.

Another aspect of the present invention relates to a mode conversiondevice comprising:

a substrate including an upper surface;

a ridge waveguide section including a first height perpendicular toupper surface of the substrate and a first width parallel to the uppersurface of the substrate;

a ridge waveguide expansion section including the first height and anexpanding width;

a tapering strip-loaded region comprising a first portion including thefirst height and a tapering width; and initial side portions, one oneach side of the first portion, including a second height, shorter thanthe first height, and an expanding width; and

an expanding strip-loaded region comprising a middle portion includingthe first height and a constant width; and final side portions includingthe second height, and an expanding width.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 is a top view of the optical device in accordance with anembodiment of the present invention;

FIG. 2 is a top view of the device of FIG. 1, including a firstprotective layer and a first etch layer;

FIG. 3 is a top view of the device of FIG. 1, including a secondprotective layer and a second etch layer;

FIG. 4 is a top view of the device of FIG. 1, including the second etchlayer overlapping the first etch layer;

FIGS. 5a and 5b is a top view of the device of FIG. 1, including slightmisalignment of the second etch layer overlapping the first etch layer;

FIGS. 6a and 6b illustrate a first step of an embodiment of the methodof the present invention;

FIGS. 7a and 7b illustrate a second step of the method of FIGS. 6a and 6b;

FIGS. 8a and 8b illustrate third and fourth steps of the method of FIGS.6a and 6b ; and

FIGS. 9a and 9b illustrate fifth and sixth steps of the method of FIGS.6a and 6 b.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art.

The present invention relates to a multi-layer optical device and methodthat addresses the aforementioned challenges. The present invention mayachieve several goals, by limiting most surfaces of the optical deviceand each portion of a hard mask to a single etch, and thereby making thedevice relatively tolerant to misalignment in the partial etch stepswith respect to the hard mask step. Using the approach of the presentinvention may result in low-loss coupling between ridge and strip-loadedwaveguides within waveguide fabrication processes.

With reference to FIG. 1, the present invention relates to a multi-layeroptical device, e.g. a mode converter 1, including a ridge waveguidesection 2, and a strip-loaded ridge waveguide section 4, all supportedon a substrate 5. The illustrated example also includes an expansionsection 3, which is not required for all embodiments. The ridgewaveguide section 2 may comprise an elongated rectangular waveguideincluding a first input/output 7 at an outer free end, and a secondinput/output 8 proximate the expansion section 3. Ideally, the ridgewaveguide section 2 includes an upper surface with a consistent heightparallel to the upper surface of the substrate 5, e.g. about 200 nm to300 nm, and substantially perpendicular edges perpendicular to the uppersurface of the substrate 5 with a width of, e.g. about 400 nm to 600 nm,typically capable of sustaining light in a single mode along alongitudinal axis 10 defining a direction of light propagation; however,other shapes, sizes and capabilities are within the scope of theinvention.

The expansion section 3 includes a ridge waveguide expansion region 11,which may have the same height as the ridge waveguide section 2, butwith a width that expands gradually, e.g. linearly or curved, from thesecond input/output 8 to an expanded width at a transition point 12. Theedge of the expansion section may expand linearly or on average at anexpansion angle θ of about 5° to 35°, ideally 10° to 15°, from thelongitudinal axis 10. The expanded width may be about twice the originalwidth, e.g. 800 nm to 1200 nm, while still sustaining a single mode. Theridge waveguide expansion region 11 is capable of converting an opticalmode when propagating between the ridge waveguide section 2 and thestrip-loaded ridge waveguide section 4.

At the transition point 12, the expansion section 3 becomes a firststrip-loaded region 13, which comprises a stepped structure, including atapering middle portion 14 a with the same height as the ridge waveguideexpansion region 11, and expanding side portions 16 a and 17 a with aheight less than the tapered middle portion 14 a, e.g. less than half orabout 100 nm to 150 nm. from the upper surface The tapering middleportion 14 a may include a width that tapers gradually, e.g. linearly orcurved, from the expanded width down to approximately the original widthof the ridge waveguide section 2, e.g. 400 nm to 600 nm. The taperingmiddle portion 14 a may taper down to a constant middle portion 14 blinearly or on average at a tapering angle α, which may be the same asthe expansion angle, e.g. about 20° to 35° from the longitudinal axis10, or at some other suitable angle.

The expanding side portions 16 a and 17 a may gradually expand linearlyor on average at the same angle as the ridge waveguide expansion region11. A substantially triangular-shaped area may be formed in theexpanding side portions 16 a and 17 a due to the side portions 16 a and17 a expanding while the middle portion 14 a taper.

In the strip loaded waveguide section 4, the constant middle portion 14b extends from the tapering middle portion 14 a with a constant heightand width, e.g. the same as ridge waveguide section 2, symmetrical aboutthe longitudinal axis 10, while final side portions 16 b and 17 bcontinue to gradually expand, e.g. at approximately the same expansionangle as the expanding side portions 16 a and 16 b, to a secondinput/output 18.

Each of the final side portions 16 b and 17 b may be 4 to 5 times widerthan the final middle portion 14 b, e.g. 1600 nm to 3000 nm, with anoverall total width of at least 8 times, and preferably 10 times thewidth of the ridge waveguide section 2, e.g. 3200 nm to 6000 nm.

The index of refraction of the ridge waveguide section 2, the expansionsection 3, and the strip-loaded ridge waveguide section 4 is higher thanthe substrate 5, which provides a lower cladding therefor. The sides andupper portions of the ridge waveguide section 2, the expansion section3, and the strip-loaded ridge waveguide section 4 may be covered with anupper cladding layer with a lower index of refraction to act as an uppercladding. Alternatively, air may provide the upper cladding. The ridgewaveguide section 2, the expansion section 3, and the strip-loaded ridgewaveguide section 4 may be comprised of a semiconductor material, e.g.silicon, and the substrate 5 may be comprised of a dielectric material,e.g. silicon dioxide. In a preferred example the substrate 5 andwaveguide sections 2, 3 and 4 are formed from a SOI structure, with thewaveguide sections 2, 3 and 4 in an upper silicon layer, and thesubstrate 5 formed of the middle silicon dioxide (BOX) layer and thebottom handle silicon layer. However, other suitable materials may beused for the waveguide sections 2, 3 and 4, and the cladding, e.g. thesubstrate 5.

Unlike conventional photonics processes, the present invention mayutilize more photoresist steps in combination with a single hard maskstep, so that ideally any particular substantial waveguide surface mayonly be exposed to a single etch step in the course of the entireprocess. Therefore, additional mask steps may be required, butwaveguides, where two etches touch the optical surface in the sameplace, are limited to designated locations, e.g. transition points 12,at which points the mode 19 has been slightly expanded in the horizontaldirection to minimize harmful effects on the light. Eliminating multipleetches lowers waveguide loss and improves repeatability.

With reference to FIGS. 2, 3 and 4, the method of the present inventionuses a plurality of different protective layers, for example a firstprotective shield, comprising one or more of a hard-mask layer 21 and afirst photoresist layer 22, and a second protective shield comprising asecond photoresist layer 23. The method also includes a plurality ofetching layers comprising at least a partial-etch layer 24 and afull-etch layer 25. Additional protective layers and etch layers arepossible, as hereinafter described, in particular when additionalstructures are provided on the substrate 5. Typically, the firstprotective shield is used to protect features that will not be etched,while the second protective shield protects features, which have alreadybeen etched, from further etching.

In the illustrated embodiment, the first and second protective shieldsboth include the hard mask layer 21, which may be comprised of a thinlayer of silicon nitride, and is applied at the start of the process, toprotect the ridge waveguide section 2 and the constant middle portion 14b of the strip-loaded ridge waveguide section 4 from being etched.Typically, the hard mask layer 21 extends the entire length of thecentral region of the mode converter 1 across the expansion section 3.Ideally, the hard mask layer 21 includes about the same constant widthas the ridge waveguide section 2 and the constant middle portion 14 b ofthe strip-loaded ridge waveguide section 4, e.g. 400 nm to 600 nm,leaving the wider portions of the expansion section 3 and tapered middleportion 14 a unprotected.

With reference to FIG. 2, the remainder of the first step comprisesplacing the rest of the first protective shield, e.g. the firstphotoresist layer 22, over the ridge waveguide section 2, the expansionregion 11, and the tapered middle portion 14 a, thereby defining thetapering edges of the tapering middle portion 14 a, and protecting thehard mask layer 21 over the ridge waveguide section 2, the expansionregion 11, and the tapering middle portion 14 a. Accordingly, the firstphotoresist layer 22 extends outwardly from either side of the hard masklayer 21 at the tapering angle α from the longitudinal axis 10. Thefirst photoresist layer 22 may also cover and protect other elements onthe substrate 5. The first photoresist layer 22 protects the parts ofthe optical device at the upper level not already protected by the hardmask layer 21, in particular parts of the optical device at the upperlevel transitioning to a part of the optical device at an intermediatelevel.

The partial-etch layer 24 is then applied over the areas to be partiallyetched, namely the expanding side portions 16 a and 17 a and the finalside portions 16 b and 17 b. The partial etch layer 24 also covers theconstant middle portion 14 b, which is protected by the hard mask 21,but not the ridge waveguide section 2 or the portions of the expansionsection 3 at the upper level, which are protected by the firstphotoresist layer 22. The partial etch layer 24 extends beyond the edgesof the final side portions 16 b and 17 b, i.e. etches an area greaterthan the final side portions 16 b and 17 b occupies, whereby the edgesof the final side portions 16 b and 17 b will not be subject to twoetching steps, i.e. the partial etch layer 24 and the full etch layer25, which occurs later, as hereinafter described. The partial etch layer24 removes the unprotected waveguide material 50 down to theintermediate level, e.g. about half of the waveguide material, more thanhalf, 100 nm to 150 nm, or ideally 130 nm+/−3.5 nm.

With reference to FIG. 3, the next step involves protecting thepartially etched sections of the device (mode converter 1), e.g. theexpanding side portions 16 a and 17 a and the final side portions 16 band 17 b, and the non-etched sections, e.g. the ridge waveguide section2, the expansion region 11, and the tapered middle portion 14 a with thesecond protective shield. The majority of the non-etched sections, e.g.the ridge waveguide section 2 and the constant middle portion 14 bcontinue to be protected by the hard mask layer 21 The partially etchedsections are protected by being covered by the second photoresist layer23, which defines the edges of the final side portions 16 b and 17 b,the outer part of the tapering side portions 16 a and 17 a, and theexpansion region 11. Accordingly, the second photoresist layer 23extends outwardly from each side of the hard mask 21 at the expansionangle θ from the longitudinal axis 10, from output 8 of the ridgewaveguide section 2 to the outer free end 18 of the strip-loaded ridgewaveguide section 4.

Next, the fully-etched layer 25 is applied, thereby clearly definingboth the partially and fully etched areas, i.e. the outer end 18 and theedges of the final side portions 16 b, along with the edges of the ridgewaveguide expansion region 11 and the ridge waveguide section 2.

Another aspect of the invention is that each section of the hard masklayer 21 may only be etched with a single etch step when possible. Thisensures maximum integrity of the hard mask layer 21 and also ensuresthat losses and non-uniformities remain at a minimum. Ideally, thepartial-etch layer 24 and the full-etch layer 25 are not applied on thesame place of the hard mask layer 21, by covering selected portions ofthe hard mask layer 21 with the first and second photoresist layers 22and 23 at different times to avoid any chance of removal of thewaveguide material, if the hard mask layer 21 is not be thick enough.

FIG. 4 illustrates the partial-etch layer 24 and the full-etch layer 25overlapping each other, whereby the first and second photoresist layers22 and 23 are carefully designed to avoid any overlapping of the firstand second etch layers 24 and 25 on hard mask layer 21. Moreover, everysubstantial side surface in the final mode converter device 1 onlyexperiences one of the two etches 24 or 25 to ensure the high quality ofthe sidewalls. Only two edges of the mode converter device, i.e. wherethe ridge waveguide expansion region 11 meets the tapering middleportion 14 a, and where the first photoresist layer 24 intersects thesecond photoresist layer 25, undergo multiple etches, and these edgeshave only minimal effect on the mode, since the mode has been slightlyexpanded by the ridge waveguide expansion region 11. Areas 31 a, 31 b,32, 33 a and 33 b represent a first area, in which the partial-etchlayer 24 is applied. A notch is created between the areas 31 a/ 33 a and31 b/33 b by the first photoresist layer 22. Areas 31 a and 31 brepresent the final side portions 17 a and 17 b of the strip-loadedridge waveguide section 4, area 32 represents the middle portion 14 b ofthe strip-loaded waveguide section 4, and areas 33 a and 33 b representexcess waveguide material removed subject to both the partial-etch andfull etch layers 24 and 25.

Areas 33 a, 33 b, 34 a, 34 b, and 35 represent a second area, in whichthe full-etch layer 25 is applied. A notch is created between the areas33 a/34 a and 33 b/34 b by the second photoresist layer 23. As above,the areas 33 a, 33 b represent the areas of the waveguide materialtotally removed by both the first and second full etches 24 and 25,while the areas 34 a and 34 b represent the areas of the waveguidematerial totally removed by the only the second full etch 25. The area35 represents the ridge waveguide section 2, protected by the hard masklayer 21 and the first photomask layer 22 (during the first partial etch24). Area 36 represents an area of intersection between the ridgewaveguide section 2 and the strip-loaded waveguide section 4, and anarea of intersection of the first photomask layer 22 and the secondphotomask layer 23, which covers the parts of the expansion section 3not subject to either etch step 24 or 25, protected partially by thehard mask layer 21 and fully by the first photomask layer 22 and thesecond photomask layer 23 in subsequent etching steps. The points ofintersection between the first and second photomask layers 22 and 23,respectively, correspond to the transition points 12, at which verticaledges are subject to both the partial and full etch layers 24 and 25,respectively.

FIGS. 5a and 5b illustrate the overlapping etch layers 24 and 25, as inFIG. 4, but with slight mask misalignment. In FIG. 5a , the partial-etchlayer 24 is laterally offset from the full-etch layer 25, with the hardmask layer 21 in place. In FIG. 5b , the partial-etch layer 24 islongitudinally offset from the full-etch layer 25 with the hard masklayer 21 in place. Either way the hard mask layer 21 does not undergotwo etch steps at any location, and no surface of the optical deviceundergoes multiple etch steps only the two aforementioned edges.

Another example of the method of the present invention is illustratedwith reference to FIGS. 6a to 9b . FIGS. 6a, 7a, 8a and 9a represent across section taken along a front of an optical device, e.g. line A-A inFIGS. 2 and 3, whereas FIGS. 6b, 7b, 8b and 9b represent a cross sectiontaken along a rear of the optical device, e.g. line BB in FIGS. 2 and 3.

With reference to FIGS. 6a and 6b , the initial step includes providinga layer of waveguide material 50 on the substrate 5. In a preferredembodiment, the substrate 5 includes a dielectric upper cladding layer51, e.g. silicon dioxide, and a lower handle layer, e.g. silicon. Asabove, the layer of waveguide material 50 may be between 150 nm and 350nm thick, preferably about 220 nm thick The next step includes applyingan initial protective shield over the portions of the waveguide materiallayer 50 to be protected during the first etching step, which in thisembodiment includes the hard mask layer 21 and hard mask layers 61 a and61 b defining other optical devices 62 a and 62 b on the substrate 5,such as grating couplers, modulators, etc. The hard mask layers 61 a and61 b may comprise silicon nitride or any other suitable hard maskmaterial.

With reference to FIGS. 7a and 7b , the initial step also includescompleting the initial protective shield by applying a photoresist layer63 over the entire optical device 1, so that the other optical devices62 a and 62 b may be formed. An initial etch layer is then applied toremove portions of the waveguide material down to a first level 71, e.g.down 50 nm to 75 nm, ideally 60 nm+/−3.5 nm, defining the other opticaldevices 62 a and 62 b which extend to the upper level of the waveguidematerial layer 50.

With reference to FIGS. 8a and 8b , the first step in forming theoptical device 1 is illustrated. In FIG. 8a , the first protectiveshield is completed by applying the first photoresist layer 22 over theexpansion section 3, the ridge waveguide section 2, and the portion ofthe hard mask layer 21 extending over those sections, as in FIG. 2. Thefirst photoresist layer 22 may also cover the other optical devices 62 aand 62 b via first photoresist layer 22′. In FIG. 8b , the partial etchlayer 24 is applied over an area including the final side portions 16 aand 16 b, but also including an area of the waveguide material layer 50beyond the final side portions. The partial etch layer 24 removes theunprotected portions of the waveguide material layer 50 down to anintermediate level 72. The constant middle portion 14 b is protectedfrom the partial etch layer 24 by the hard mask layer 22. The partialetch layer 24 removes the unprotected waveguide material 50 down to theintermediate level, e.g. about half of the height of the waveguidematerial 50, more than half, 100 nm to 200 nm, or ideally 130 nm+/−3.5nm, leaving the intermediate level height at about half or less thanhalf of the upper level height, e.g. between 50 nm and 175 nm.

With reference to FIG. 9b , the next step includes applying the secondprotective shield, including the second photoresist layer 23 over thefinal side portions 17 a and 17 b and the constant middle portion 14 b,as well as the portion of the hard mask layer 21 over those portions.The second protective shield may also include the second photoresistlayer 23′ covering the other optical devices 62 a and 62 b.

The full etch layer 25 is then applied, see FIGS. 9a and 9b , down tothe lower level 73, defined by an upper surface of the substrate 5 orsome other etch stop layer provide thereon. The full etch step removesthe extra waveguide material at the intermediate level 72 from eitherside of the final side portions 17 a and 17 b, as well as defining theedges of the ridge waveguide section 2 and the expansion section 3,whereby all of the vertical surfaces of the waveguide material 50 areonly subject to a single etch step.

FIGS. 9a and 9b also illustrate mode sizes of the light travelling inthe ridge waveguide section 2 and the strip-loaded waveguide region 4,in particular the expansion of the mode side from single mode 81 to aslightly wider single mode 82 then back to a single mode 81. Asdisclosed, none of the side surfaces of the mode converter 1 that areexposed to the optical mode undergo more than one etching step, only thetwo edges 12, where the ridge waveguide expansion region 11 meets thetapering middle portion 14 a, and where the first and second photoresistlayers 22 and 23 intersect, at which points the mode has been onlyslightly expanded to minimize harmful effects.

Further, in the mode conversion device 1 shown in FIG. 1, the region inwhich the multiple etches overlap, i.e. the edges 12, is extremely shortin the illustrated, on the order of 1 to 2 um in length, and hence thedistance over which the interface effects can cause losses is verysmall. In most other implementations of waveguide manufacturingprocesses, as noted elsewhere, interface effects would have many mm ofwaveguide length to build up over.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

1. A method of manufacturing a multi-layer optical device on a substrateincluding a waveguide material thereon, the optical device including aridge waveguide section comprising waveguide material extending to anupper level, and a strip-loaded ridge waveguide section comprisingportions of waveguide material extending to an intermediate level andportions of waveguide material extending to the upper level andextending from the ridge waveguide section, the method comprising: a)providing the substrate with a layer of the waveguide material thereonextending to the upper level; b) applying a first protective shield overa first area of the waveguide material to define the portions ofwaveguide material extending to the upper level of the strip-loadedridge waveguide section and the ridge waveguide section; c) applying afirst partial etch over a second area of the waveguide material, whichincludes the portions extending to the intermediate level, to etch thesecond area wider than the portions of waveguide material extending tothe intermediate level to form the portions of waveguide materialextending to the upper level of the strip-loaded ridge waveguidesection; d) applying a second protective shield over a third area of thewaveguide material defining the portions of the waveguide materialextending to the intermediate level to protect them from furtheretching, and the portion of waveguide material extending to the upperlevel of the strip-loaded ridge waveguide section and the ridgewaveguide section; and e) applying a second full etch, deeper than thefirst partial etch, over a fourth area of the waveguide material to formthe ridge waveguide section and the portions of waveguide materialextending to the intermediate level of the strip-loaded ridge waveguidesection; wherein the first protective shield and the second protectiveshield cover an area of intersection between the ridge waveguide sectionand the strip-loaded waveguide section, whereby side surfaces of thestrip-loaded ridge waveguide section and the ridge waveguide section areonly subject to a single etch, except at intersecting edges where theridge waveguide section intersects the strip-loaded waveguide section.2. The method according to claim 1, wherein steps b) and d) each includeapplying a same hard mask layer over the portions at the upper level ofthe strip-loaded ridge waveguide section and the ridge waveguidesection.
 3. The method according to claim 2, wherein in step b) thefirst protective shield also includes a first photoresist layer over theridge waveguide section, preventing etching material from being appliedover the hard mask layer on the ridge waveguide section; wherein in stepd) the second protective shield also includes a second photoresist layerover the strip-loaded ridge waveguide section preventing etchingmaterial from being applied over the hard mask layer on the portions ofthe strip-loaded ridge waveguide section at the upper level; whereby nopart of the hard mask layer is subject to two etching steps.
 4. Themethod according to claim 1, wherein the multilayer optical devicecomprises an expansion section between the ridge waveguide section andthe strip-loaded ridge waveguide section, the expansion sectionincluding a width expanding from the ridge waveguide section to thestrip-loaded ridge waveguide section capable of converting an opticalmode when propagating between the ridge waveguide section and thestrip-loaded ridge waveguide section; wherein the expansion sectioncomprises portions at the upper level and portions at the intermediatelevel; and wherein applying the first protective shield also includesapplying the first protective shield to the portions of the expansionsection at the upper level, while excluding portions of the expansionsection at the intermediate level.
 5. The method according to claim 4,wherein applying the second protective layer comprises applying thesecond protective shield over the portions of the expansion section atthe upper and intermediate levels.
 6. The method according to claim 5,wherein the strip-loaded ridge waveguide section comprises: a middleportion including a height at the upper level and a width substantiallythe same as the ridge waveguide section; and at least one side portionincluding a height at the intermediate level and width that expandsoutwardly from a side of the middle portion.
 7. The method according toclaim 6, wherein the expansion section comprises: a ridge waveguideexpansion region including a height at the upper level and a widthexpanding outwardly from the ridge waveguide section; and an expandingstrip-loaded region including: a tapering middle portion including aheight at the upper level and a width tapering down to the middleportion of the strip-loaded ridge waveguide, and expanding side portionsincluding a height at the intermediate level and a width expandingoutwardly from each side of the tapering middle portion to meet the sideportions of the strip-loaded waveguide section.
 8. The method accordingto claim 7, wherein the first protective shield extends along an edge ofthe tapering middle portion of the expanding strip-loaded region oneither side thereof, thereby defining the expanding side portions of theexpansion section at the intermediate level and protecting the ridgewaveguide expansion region and the tapering middle portion of theexpanding strip-loaded region during the first partial etch.
 9. Themethod according to claim 8, wherein the second protective shieldextends along an edge of the expanding strip-loaded region and along anedge of the expanding side portions of the expansion section on eitherside thereof, thereby defining the expanding strip-loaded region and theexpanding side portions of the expansion section during the second fulletch.
 10. The method according to claim 1, further comprising: applyinga primary protective layer over an area of the waveguide materialdesignated for or including the waveguide device; applying a secondaryprotective layer over an area of the waveguide material designate for anadditional waveguide feature; and applying a partial etch around thesecondary protective layer to define the additional waveguide features.11-20. (canceled)